Some diesel engines suffer from a problem called "hot start knock" and "hot start stall" as described in the background section of U.S. Pat. No. 5,159,911 to Williams et al. and assigned to the same assignee, Cummins Engine Company, Inc., as the subject application. In particular, multi-cylinder diesel engines equipped with open nozzle unit injectors of the type disclosed in U.S. Pat. Nos. 3,351,288 and 3,544,008 can experience "hot start knock" or "hot start stall" as a result of diesel fuel leaking into some of the engine cylinders. The severity of the problem is dependent primarily on starting system capability, engine temperature, type of fuel and compression ratio. An associated problem can be excessive smoke and noise even if start up is successful.
During cranking after a hot shutdown, subsequent restarts can be accompanied by a loud series of knocks or stalls which may hurt the starting circuit. Thus, hot start knock or hard starting occurs when an engine is keyed off and restarted when warm (under 15 minutes). Several loud bangs may be heard (hot start knock), or the engine may be hard to start or fail to turn over (hard starting or cranking stall). The root cause of the problem is that a very small quantity of fuel (about 10 drops or 15 milliliters) dribbles, from the metering side of the fuel system, into the combustion chamber of one or two cylinders at hot shutdown. In a typical 6 cylinder combustion ignition engine equipped with open nozzle unit injectors, two cylinders are open at shutdown, allowing this small quantity of fuel to fall into their respective piston bowls. Under cold start conditions, this fuel condenses and is burned off after the engine is restarted. However, under hot shutdown conditions, this fuel vaporizes and creates a combustible mixture which remains in the combustion chamber. Overhead fuel tanks, drain line check valves or excessive drain line restriction seem to make this problem worse. While there is no evidence that this problem shortens engine life or causes permanent engine damage nor detrimental to engine performance, reliability or durability, it is, however, annoying and can lead to some starter motor difficulties if the starter torque is insufficient to crank the engine through the hot start knock events.
While there are devices, such as normally open fuel shut off valves (FSOV), which are effective for eliminating hot start knock on non-overhead tank applications, there is no device available to eliminate hot start knock on overhead tank applications. A normally open FSOV, when de-energized during shutdown, would permit fuel to flow from the higher fuel tank to the engine and cause hydraulic lockup of several cylinders. This has been a limiting factor in applying existing countermeasures to overhead tank applications having vertical tanks. In the non-overhead tank application, the normally open fuel shut off valve is connected between the rail tee and gear pump suction. At key-off the valve opens and allows about 10-15 ml of fuel to vent back to gear pump suction and to the fuel tank. It has been discovered that the normally open valve can be misapplied and has an inherent problem that a non-vented or overhead tank can cause hydraulic lockup of the engine. For example, some engine installations such as marine or over the road vehicle may take on an effective "overhead" fuel tank configuration if the tank is full and the engine installation is inclined by more than 30.degree.. A delay algorithm may be used to delay FSOV opening until during cranking (not immediately at key-on) to allow timing rail to vent to timing circuit during cranking (working above 100 rpm, 100.degree. F. coolant temp).
However, the normally open FSOV approach creates concerns about misapplication, angularity, and robustness and may not open against more than a 2 psi pressure difference across the valve.
One attempt to solve the "hot start" problem is disclosed in commonly assigned U.S. Pat. No. 5,159,911 to Williams et al. wherein a fuel leakage prevention system is disclosed including a vacuum chamber which is adapted to be placed in fluid communication with the common rail upon engine shut down. Another approach is disclosed in U.S. patent application Ser. No. 08/899,318 filed Aug. 13, 1997 (also assigned to Cummins Engine Company, Inc.) which discloses a variable volume chamber in fluid communication with the common rail of an engine wherein upon engine shut down the variable volume chamber is allowed to expand and thereby remove fuel from the common rail. While both of these approaches have some advantages, factors such as high relative cost, complexity, difficulty in retrofitting, reliability, and efficacy cause these solutions to be less than ideal.
A need therefor still exists for a "hot start" solution that is simple, highly reliable, easily installed either on original equipment or as a retrofit and effective.